Method of mounting heat sinks

ABSTRACT

Disclosed is a compensating roll pin securing a heat sink to a circuit board. The roll pin comprises a deformable body of resilient material, such as annealed spring steel, formed into a generally cylindrical body having first and second end portions separated by a middle portion of enlarged external diameter and an axially extending gap in the surface of the body. A method of use is also disclosed wherein the first end portion of the roll pin is inserted into an aperture in a heat sink and the middle portion deformed within the aperture to form a tight fit between the pin and heat sink. A substantial part of the middle portion remains outside the aperture in the heat sink. The second end portion is inserted into a hole in a circuit board and forms an interference fit therewith. The middle portion forms a stop to prevent further insertion of the roll pin into the circuit board so that a gap of predetermined distance separates the heat sink and circuit board to enhance cooling.

This application is a division of application Ser. No. 851,517 fieldApr. 14, 1986 entitled Compensating Roll Pin for Heat Sink Mountingwhich is a division of application Ser. No. 644,215 filed Aug. 24, 1984entitled Compensating Roll Pin for Heat Sink Mounting which issued asU.S. Pat. No. 4,602,315 on July 22, 1986.

This invention relates to mounting of electrical components,particularly heat sinks, on a circuit board.

Modern electronic devices, such as transistors, often generateconsiderable heat. If provisions are not made to cool the device, thedevice can exceed its operating temperature and fail. In addition,adequate cooling of a device can lengthen its service life and increasereliability.

A common technique for cooling an electrical device is the use of a heatsink. The electronic device is mounted on the heat sink so that the heatgenerated in the device is readily transferred to the heat sink. Theheat sink has a configuration, including cooling fins, which provideseffective dissipation of heat from the heat sink to the atmosphere,either by natural convection or forced convection by use of a coolingfan.

Electronic devices are typically employed within a circuit laid out on acircuit board. If the electronic device is mounted on a heat sink, theheat sink will, in turn, be mounted to the circuit board with thenecessary electrical connections being made between the device and thecircuit board.

In the past, solid roll pins have been employed to mount a heat sink ona circuit board. The heat sink will be formed with an aperture ofdiameter equal to or slightly less than the diameter of the roll pin foran interference fit between the roll pin and the heat sink. A hole willbe formed on the circuit board which also has a diameter equal to orless than the diameter of the roll pin so that the roll pin will alsohave an interference fit with the circuit board. Unfortunately, the useof such solid roll pins requires extremely fine tolerances in the heatsink and circuit board which are often difficult and expensive toachieve. Furthermore, if either the hole in the circuit board oraperture in the heat sink are enlarged through wear, the heat sink willno longer be securely mounted to the circuit board.

The CEM Company has introduced a roll pin marketed under the trademarkSPIROL. This pin is formed by rolling a sheet of material into a tighthelix, thereby resembling a cylinder. The helix can extend two completerevolutions about its center axis. While this is an improvement overconventional solid roll pins, the SPIROL roll pins will frequently betoo large in diameter or too small in diameter which either makes itdifficult to mount the heat sink on the circuit board or prevents theheat sink from being securely mounted thereon.

In addition to simply mounting the heat sink on a circuit board, it isoften desirable to space the heat sink from the board to permit free airflow past the bottom of the heat sink. With the conventional roll pinsnoted above, it is very difficult to position the heat sink at a precisedistance away from the circuit board using the pins alone.

A need therefore exists for a roll pin having an enhanced ability tosecure a heat sink to a circuit board with less emphasis on precisecontrol of the diameter of the hole in the circuit board and theaperture in the heat sink. Furthermore, a need exists for a roll pinwhich permits precise positioning of the heat sink at a spaced distancefrom the circuit board.

In accordance with the present invention, a roll pin is provided formounting a heat sink on a circuit board. The circuit board has at leastone hole formed therein of first predetermined diameter. The heat sinkhas at least one aperture therein having a semicircular cross-section ofa second predetermined diameter. The roll pin comprises a deformablematerial which is rolled into a generally cylindrical shape with a gapbetween the facing edges of the material. The pin has first and secondend portions and a middle portion. The middle portion has a greaterexernal diameter than the end portions. The gap between the facing edgesin the middle portion is larger than the gap betwen the facing edges ofthe first and second end portions.

The first end is slidable into the aperture in the heat sink and themiddle portion is deformed as it is urged into the aperture to provide atight fit between the roll pin and heat sink. A substantial part of themiddle portion remains external of the aperture. The second end portionis slidable into the hole in the circuit board for an interference fittherewith until the middle portion contacts the circuit board and spacesthe heat sink a predetermined distance from the circuit board.

In accordance with another aspect of the present invention, a method isprovided for mounting a heat sink on a circuit board. The methodincludes the step of sliding the first end of a roll pin into anaperture formed in the heat sink, the roll pin having first and secondend portions and a middle portion, the middle portion having a greaterexternal diameter than the end portions, the roll pin being formed byrolling a deformable material into a generally cylindrical shape with agap between the facing edges of the material. The method furtherincludes the step of urging the middle portion into the aperture todeform the middle portion and provide a tight fit between the roll pinand the heat sink with a substantial part of the middle portion externalof the aperture. The method further includes the step of sliding thesecond end of the roll pin into a hole in the circuit board for aninterference fit with the board until the middle portion contacts thecircuit board to space the heat sink a predetermined distance from thecircuit board.

For a more complete understanding of the invention and the advantagesthereof, reference is now made to the following detailed descriptiontaken in connection with the accompanying drawings in which:

FIG. 1 is a side view of a roll pin forming a first embodiment of theinvention;

FIG. 2 is an end view of the roll pin of FIG. 1;

FIG. 3 is an exploded perspective view of a heat sink and circuit boardand the roll pins used to secure the heat sink to the circuit board; and

FIG. 4 is a side view, in partial cross-section, of the heat sinkmounted on the circuit board with the roll pin.

Referring simultaneously to FIGS. 1-4, a compensating roll pin 10constructed in accordance with the present invention is illustrated. Theroll pin 10 is employed to secure a heat sink 12 to a circuit board 14as seen in FIGS. 3 and 4. Furthermore, the roll pin 10 serves as aspacer to space the heat sink 12 a predetermined distance 16 above thesurface of the circuit board 14 to permit air to flow along the bottom18 of the heat sink 12.

The heat sink 12 has a number of holes 20 formed therein. Holes 20 areadapted to receive some type of electrical component, such as atransistor, for mounting on the heat sink. The heat generated by theelectrical component will be conducted into the heat sink 12 anddissipated to the surrounding atmosphere. While the main structuralsupport for the electronic device will be the heat sink, various leadsfrom the electronic device will be directly connected to the circuitboard 14.

As can best be seen in FIGS. 1 and 2, the roll pin 10 is formed byrolling a deformable material into a generally cylindrical shape. Theroll pin 10 includes a first end portion 22, a middle portion 24 and asecond end portion 26. As the material is rolled, the facing edges 28and 30 will be separated by a gap 32 which will permit the pin to deformslightly inwardly to decrease its external diameter as will be discussedin greater detail hereinafter.

As can best be seen in FIG. 3, the heat sink 12 includes apertures 34which form an integral part of the cooling fin structure at each end ofthe heat sink. The apertures 34 have a generally semicylindricalcross-section and a first predetermined diameter 36. The circularcross-section of aperture 34 extends about an arc of over 180° but canbe less than 360°.

The circuit board 14 has a pair of holes 38 formed through the board forreceiving pins 10 secured to the heat sink 12.

When the heat sink 12 is to be mounted on the circuit board, the rollpins 10 are preferably first mounted in the heat sink 12. For each pin10, the first end portion 22 is slid into an aperture 34 of heat sink 12at the bottom 18 until the middle portion 24 contacts the opening in theaperture 34. The outer diameter of the first end portion 22 can bedesigned to either provide some interference fit between the first endportion and the aperture 34 or to permit the first end portion 22 toslide without interference into the aperture 34. A conical guide surface39 is provided on the first end portion 22 to assist in inserting itwithin the aperture 34.

When the middle portion 24 is moved into contact with the opening ofaperture 34, the pin 10 can be forced into the aperture 34. This willdeform a part of the middle portion 24 to provide a tight fit betweenthe pin 10 and the heat sink 12 as seen in FIG. 4. As can best be seenin FIG. 1, the gap 32 in the middle portion 24 is significantly largerthan the gap in the end portions 22 and 26. This permits the middleportion 24 to be deformed radially inwardly, reducing the gap 32, as themiddle portion is inserted within the aperture 34. As will be observedin FIG. 4, only part of the middle portion 24 will need to be insertedin the aperture to provide a tight fit in the heat sink 12 so that asignificant part of the portion 24 remains outside the aperture 34.

When the roll pins 10 have been properly mounted in the heat sink 12,the second end portion 26 of the roll pins 10 can be inserted within theholes 38 in the circuit board. The second end portion also has a conicalguide 40 to assist the entry of the portion 26 within the hole 20. Theexternal diameter of the second end portion 26 is preferably sized sothat an interference fit is created between portion 26 and hole 38 tosecure the roll pins 10 and heat sink 12 on the circuit board 14. Thegap 32 in second end portion 26 will permit the diameter of hole 38 tobe less precisely formed. The external diameter of the end portion 26can descrease to compensate for the size of hole 38 as the pin 10 isinserted in hole 38 through deformation of end portion 26 permitted bygap 32 while still providing a tight interference fit. This feature alsowill permit the heat sink 12 to be securely mounted to board 14 even ifthe diameters of holes 38 are increased by wear caused by removal andremounting of the heat sink 12. The pin 10 can be constructed ofmaterial of sufficient resiliency so that the end portion 26 will expandto near its original undeformed external diameter. Thus, if holes 38 donot wear to a diameter greater than that of the non-compressed endportion 26, the pins 10 will provide repeated secure mounting for heatsink 12.

In the preferred embodiment the second end portion 26 is urged into ahole 38 so that the middle portion 24 comes into contact with the uppersurface 42 of the circuit board. Because a finite length of the middleportion 24 is outside the aperture 34, the heat sink 12 will be spacedthe predetermined distance 16 off the upper surface 42 of the circuitboard to permit cooling air to flow to all sides of the heat sink.

The conical transition surface 44 between the first end portion 22 andmiddle portion 24 acts as a wedge to begin the deformation of the middleportion 24 as it is forced into the aperture 34. The conical transitionsurface 46 between the second end portion 26 and middle portion 24 actsas a stop to limit the extent to which pin 10 can be inserted into ahole 38 in the circuit board 14.

The heat sink will often need to be electrically connected to thecircuit board 14. Commonly, the heat sink will be grounded to thecircuit board ground. Preferably, the roll pins 10 which mount the heatsink 12 on the circuit board 14 will also be used as the electricalconnection between these two members. For this reason, the roll pin 10should readily accept a solder bond. The roll pin 10 can be clad with asolderable coating material which promotes the formation of solder bondssuch as tin or tin-lead alloys. The pre-clad material may be applied byconventional pre-tinning or by plating or the like. Such pre-clad isconventionally referred to as "pre-tin" regardless of the composition ofthe material or the process by which it is applied, and the terms"pre-tinned", "pre-tin", "tin-plated" and "tin-coated" areinterchangeably used to refer to solderable or solder promoting coatingsregardless of the actual composition of the coating or the manner bywhich the coating is applied.

While any suitable material can be used to form a roll pin 10, roll pins10 constructed from annealed spring steel having a thickness of 0.015in. (0.38 millimeters) have proven satisfactory. The external diametersof the first and second end portions are identical at 0.085 in. (2.16millimeters) and the difference in diameter between the middle portionand the end portions was about 0.020 in. (0.51 millimeters).

As can be seen, the roll pin 10 has several advantages over the priorart. Initially, since the middle portion 24 will be deformed to enterthe aperture 34, the permitted variance in the nominal diameter of thepin will be greater than if a solid roll pin is used for an interferencefit with the aperture. In addition, provision of the gap 32 in thesecond end portion 26 permits the dimensional tolerances in holes 38 inthe circuit board 14 to be greater as the diameter of the second endportion can be varied to fit the holes 38 by deforming the end portion26 and reducing the size of the gap 32. Furthermore, middle portion 24provides a simple and accurate means for spacing the heat sink a desireddistance 16 from the circuit board 14 without the use of additionalspacing members which increase the cost and complexity of the heat sink.In the preferred embodiment, the roll pin 10 will be symmetrical aboutthe middle portion 24 which permits each end portion of the pin to beinserted in either the heat sink or the circuit board.

While the invention has been described herein with respect to a specificembodiment thereof, it will be understood that various changes andmodifications will be suggested to one skilled in the art and it isintended that such changes and modifications fall within the scope ofthe appended claims.

What is claimed is:
 1. The method of mounting a heat sink having anaperture of first predetermined diameter therein on a circuit boardhaving a hole of second predetermined diameter therein comprising stepsof:(a) inserting the first end of a roll pin into said aperture, saidroll pin comprising an elongated hollow cylindrical body having anaxially extending gap in the surface thereof and a first end portion, asecond end portion and middle portion intermediate said first and secondend portions with the external diameter of said middle portion largerthan the external diameter of said end portions; (b) urging the middleportion of said roll pin into the aperture sufficiently to cause saidmiddle portion to deform and provide a tight fit between the roll pinand the heat sink with a substantial part of the middle portionextending from said aperture; and (c) inserting the second end portionof said roll pin into said hole in the circuit board and forming aninterference fit with said hole in the board when said middle portion ofsaid roll pin contacts the circuit board, thereby spacing the heat sinka predetermined distance from the circuit board.
 2. The method of claim1 further comprising the step of guiding the roll pin into the aperturein the heat sink and the hole in the circuit board with conical guidesurfaces formed on the external surfaces of said first and second endportions of said roll pin.
 3. The method of claim 1 further includingthe steps of:(a) initiating deformation of the middle portion of theroll pin by contact between a conical transition surface on the roll pinbetween the first end portion and the middle portion thereof and thewalls defining the aperture in said heat sink; and (b) stopping theinsertion of the second end portion of the roll pin into the hole in thecircuit board with a conical transition surface between the second endportion and the middle portion.
 4. The method of claim 1 furtherincluding the step of coating the roll pin with solderable material topromote the formation of a solder bond